The present invention relates to a bank formation in which a sheet formed of a plastic material such as plastics, rubber or the like is formed by passing a gap between opposing two rolls under being applied by a nipping pressure therebetween while forming a bank of the plastic material on an upstream side of the gap between the two rolls, and more particularly, to a method of controlling a sheet profile for the bank formation so as to provide a sheet having a uniform thickness by changing a bank profile in a range of the bank amount in which any bank mark due to the excessive bank amount is prevented from causing and any touch fault due to the less bank amount is also prevented from causing.
In a conventional bank formation of such art field, when a sheet having relatively large thickness is formed, there is adopted a method, for the purpose of improving the surface condition of the sheet to be formed, of supplying, and then rolling, a plastic material, which is heated and plasticized and is pushed out in sheet shape through a discharge port of a die, between two formation rolls. In this method, since the sheet is subjected to rolling and transferring functions by the two formation rolls, the surface brightness and the surface smoothness of the sheet can be improved, and a bank of a plastic material such as fused resin is formed on the upstream side of the gap between the two formation rolls. Further, the term "upstream" referred to herein is used with reference to the flow of the fused material or sheet.
In such sheet formation method for forming the bank, called hereinafter bank formation or bank formation method, a fault is caused on the surface of the sheet formed in the case of excessive amount of the bank or in the case of less amount thereof. Accordingly, it is required to adjust the bank amount in a range not to cause any fault, and moreover, it is also required to adjust the sheet thickness in a predetermined allowable range in accordance with objects or conditions for use. That is, in the bank formation, it is important to form the bank while suitably maintaining the bank amount and uniformly keeping the thickness of the sheet to be formed.
The applicant of the subject application has provided the following two methods for solving the above mentioned problems in the conventional method such as disclosed in the Japanese Patent Publication No. HEI 6-37065.
The first method is one for controlling or adjusting a discharge amount from a die to make uniform the bank amount in the cross direction thereof.
In this method, however, although the bank profile can be controlled, the sheet profile cannot be controlled because of no consideration of data concerning the sheet thickness. That is, the central portion of the sheet profile is made thickened because of deflection of the rolls. Furthermore, in a case where the gap between the two rolls provides an oblique shape, the formed sheet profile also has an oblique shape.
The second method is one for controlling the sheet profile by causing a traverse flow in the cross direction of a plastic material at the bank portion through the adjustment of the gap between the two rolls.
In this method, however, since the gap is adjusted by forcibly bending the rolls, it is impossible to adjust or remedy a sheet profile having small pitches of recessed and protruded portions and irregular thickness.
Furthermore, there has been further provided a conventional ordinary method, as a third method, in which a thickness gauge for measuring the thickness of the sheet is arranged and the discharge amount of the die is controlled by feeding back the thickness data measured by the thickness gauge, thereby maintaining constant the thickness of the sheet.
In this third method, however, although the thickness of the sheet can be controlled in a case of less traverse flow of the plastic material at the bank portion, much time is required till the controlling has been completed because the movement of the plastic material from the die to the thickness gauge takes a time of 10 to 30 minutes. Moreover, in this method, since the controlling cannot be adapted immediately to the change of the sheet thickness due to disturbance such as temperature change of the die, the possibility of providing products having faults will be increased. Still furthermore, since the bank amount is not considered, the surface of the sheet may be damaged.